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Southwest California area forecast discussion 
National Weather Service Los Angeles/Oxnard California
1030 PM PDT Wednesday may 6 2015

..updated aviation discussion...

Synopsis...

A cold upper level low pressure system will bring a chance of
showers and thunderstorms along with below normal temperatures to
the area Thursday through Friday. Snow levels may drop to 5000 feet.
High pressure will bring warmer temperatures to the area over the
weekend and into early next week.

&&

Short term (tonight-sat)...early this evening...plenty of stratocu
covered the central coast...the vtu coast/vlys/cstl slopes...and the
l.A. County valleys and coastal slopes as strong onshore gradients persist
(+7.6 mb lax-dag at 02z). The stratocu will expand through the night
and cover just about all coastal and valley areas including much of the
slo interior valleys...and the mountain slopes up to 6000 feet or so. A few
light showers or sprinkles will be possible overnight for much of
l.A. County except for the Antelope Valley...and also for the southeastern valleys
of vtu County. In the Antelope Valley...gusty west to SW winds up to 45
miles per hour lingered...with gusts up to 55 miles per hour around Lake Palmdale. The
winds will drop below advisory levels overnight but remain quite
gusty at times. A Wind Advisory is in effect for the Antelope Valley
through Thursday evening as winds should increase and become strong again
on Thursday.

A broad upper level trough will linger over California tonight while an upper
low deepens north of the Bay area late tonight. This cold upper low will
move S to near slo County by Thursday afternoon then drift off to the east
through Friday...reaching southern Nevada by late in the day. The upper low will
continue to move east Friday night and Sat...while an upper level ridge
over the east Pacific slowly builds toward the California CST. A broad northwest flow
aloft will prevail over southwestern California Friday night and Sat.

The upper low...along with a cold and unstable airmass and 500 mb
temperatures around -23 to -25 degree c over southwestern California...will bring cool
temperatures and a chance of showers to much of the forecast area by
Thursday afternoon. There will also be a slight chance of thunderstorms
for the mountains and much of slo/sba counties by Thursday afternoon. The
slight chance to chance of showers along with a slight chance of
thunderstorms will then persist across much of the region Thursday night
and Friday. A slight chance of showers and thunderstorms should
continue mainly Friday evening in the mountains and eastern l.A. County. Any
thunderstorms that form will be capable of producing brief heavy
rain...gusty winds...small hail and dangerous cloud-to-ground
lightning. This will be a very showery weather system...and rainfall
amounts will be highly variable. However...it appears that from Thursday
through Friday evening...rainfall of around 0.10 to 0.33 inch can be
expected for the coast and valleys...and 0.25 to 0.75 inch for the
mountains...with local amounts up to an inch or more not out of the
question in the mountains some areas may see little or no rainfall.

Snow levels will lower to near 5000 feet Thursday afternoon and Thursday night
before increasing to 5500 to 6000 feet on Friday. Local snowfall
accumulation of up to 4 inches cannot be ruled out with this weather
system above about 5500 to 6000 feet. Gusty winds up to 45 miles per hour can
also be expected the mountains later Thursday and Thursday night. The combination
of gusty winds and some accumulating snowfall may prompt the
issuance of a Winter Weather Advisory as we draw closer to the event
as wintry driving conditions will be possible up to resort levels.

There may be a few lingering clouds over the area on Sat...otherwise
mostly sunny skies should prevail by Sat afternoon along with warmer
temperatures in all areas.

Temperatures are forecast to be significantly below normal Thursday and Friday...
then warm to slightly below normal for most areas on Sat.

***From previous discussion***

Long term (sun-wed)...

Temperatures will return to near normal Sunday as an upper-level
ridge moves overhead. Long range models agree that the upper ridge
will be short-lived as another trough of low pressure develops early
next week and persists through Wednesday. Despite the trough...near
normal temperatures are expected Monday through
Wednesday...however...there is a good chance that may gray
conditions will return during this time.

&&

Aviation...07/0530z...

At 0400z...the marine inversion at klax was 5800 feet. The top of
the inversion was 7400 feet with a temperature of 7 degrees celsius.

Overall...moderate confidence in 06z taf package. North of Point
Conception...a lower inversion is in place with widespread MVFR ceilings
at taf sites. High confidence in these MVFR ceilings continuing through
Thursday morning...before scattering out to a VFR deck Thursday
afternoon. With upper low approaching...there may be some scattered
showers Thursday afternoon/evening...so will include some vcsh
remarks with the 06z taf package. Also...there is a 10-15% chance of
some isolated thunderstorms...but too low confidence to include any
mention in current taf.

South of Point Conception...inversion is deeper and moderately
confident that ceilings will remain at VFR levels. There is a 30% chance
that a lower inversion may develop overnight...producing some MVFR
ceilings at coastal/valley taf sites. Any MVFR ceilings that develop
overnight should scatter out to the VFR cloud deck Thursday
afternoon. By Thursday evening...there is the possibility of some
scattered showers developing...and will include a vcsh remark after
02z. Again...there is about a 5-10% chance of a thunderstorm Thursday evening
but too low of a percentage to include in taf at this time.

Klax...moderate confidence in 06z taf. There is a 30% chance of a
broken MVFR deck developing overnight (10z-20z). By Thursday evening
(after 02z) there is the possibility of some showers and a very
slight chance (5-10%) of a thunderstorm.

Kbur...moderate confidence in 06z taf. There is a 30% chance of a
broken MVFR deck developing overnight (10z-20z). By Thursday evening
(after 02z) tehre is the possibility of some showers and a very
slight chance (5-10%) of a thunderstorm.

&&

Marine...06/800 PM.

For the outer waters...high confidence in northwest winds remaining
at Small Craft Advisory levels through Thursday evening. The winds
will diminish to below advisory levels on Friday and Saturday but
will strengthen again on Sunday with a chance of reaching Small
Craft Advisory levels.

For the inner waters...
north of Point Conception there is moderate confidence in northwest
winds reaching Small Craft Advisory levels this evening then the
wind will diminish through the weekend.

South of Point Conception there is high confidence in west to
northwest winds remaining below Small Craft Advisory levels through
the weekend although there is a 30% chance of periodic small craft
level gusts across the western half of the Santa Barbara Channel.

Overall... there is a slight chance of scattered showers and
isolated thunderstorms across the coastal waters on Thursday and
Friday moving down from the north. Any thunderstorms would be
capable of producing brief heavy rain...gusty and erratic winds...
rough seas...and possible waterspouts.

&&

Lox watches/warnings/advisories...
Wind Advisory (see laxnpwlox).
Small Craft Advisory (see laxmwwlox).
High surf advisory (see laxcfwlox).
Special Weather Statement (see laxspslox).

&&

$$

Public...sirard/sukup
aviation...rat
marine...kj/sirard
synopsis...b/sirard

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